A blue-color light-emitting element or ultraviolet light-emitting element can be used as a white light source if combined with an appropriate wavelength conversion material. Active studies have been conducted on applications of such white light sources to backlights for liquid crystal displays and the like, light-emitting diode illumination, automotive lighting, general lighting to replace fluorescent lighting, and so on. Some of the studies have already been put to practical use. Today, such blue-color light-emitting elements and ultraviolet light-emitting elements are produced mainly by growing a thin film of gallium nitride-based semiconductor crystal using a technique such as a metal-organic chemical vapor deposition process (MOCVD process) or molecular beam epitaxy process (MBE process) and are collectively referred to as gallium nitride-based light-emitting diodes or GaN-based LEDs.
Conventionally, most of the substrates used for GaN-based LEDs are sapphire substrates. Since sapphire and GaN differ greatly in lattice constant, a considerable dislocation on the order of 109/cm2 is unavoidable for a GaN crystal epitaxially grown on a sapphire substrate. However, sapphire substrates have the advantage of being more inexpensive than SiC substrates and GaN substrates. Moreover, light-emitting efficiencies of InGaN in blue-color light-emitting regions normally used as quantum well layers of GaN-based LEDs are insufficiently sensitive to dislocation density. For these reasons, sapphire substrates are still used widely as primary substrates under the present circumstances.
However, when gallium nitride-based semiconductor crystals are viewed as a material for devices used under conditions of high carrier density, high dislocation density such as described above considerably deteriorates device characteristics. For example, high dislocation density remarkably reduces the life of devices such as high-power LEDs or lasers. Also, when an active layer contains no In (e.g., when an AlGaN is used for an active layer) or when an active layer structure includes an InGaN layer or InAlGaN layer with a relatively small In content (e.g., about 0.2 or below) to realize light emission with a wavelength shorter than somewhere around the near-ultraviolet region, the dependence of internal quantum efficiency on the dislocation density increases, and consequently luminous intensity itself decreases if the dislocation density is high.
That is, when an active layer contains no In or when an active layer structure includes an InGaN layer or InAlGaN layer with a relatively small In content, a lower dislocation density is required more strongly than when the active layer structure includes an InGaN layer which has a emission wavelength longer than that of blue color.
When a lower dislocation density is required in this way, it is useful to adopt a GaN substrate as a substrate for epitaxial growth. It is expected that this will reduce the dislocation density found in epitaxial layers to 108/cm2 or below, or even 107/cm2 or below. Furthermore, if dislocations and the like on the substrate and the like are reduced, the dislocation density is expected to be reduced to even 106/cm2 or below. That is, the dislocation density is expected to be reduced by two to three orders of magnitude or more compared to when a sapphire substrate is used. In view of these circumstances, freestanding GaN substrates and freestanding AlN substrates are suitable as substrates for epitaxial growth of gallium nitride-based semiconductor crystals.
In most of conventional attempts to epitaxially grow gallium nitride-based semiconductor crystals on GaN substrates which are nitride substrates, c-plane ((0001)-plane) substrates are used to provide epitaxial growth surfaces, i.e., epitaxial growth takes place on a “polar plane.” Examples of reports on such attempts include Patent Document 1 (Japanese Patent Laid-Open No. 2005-347494), Patent Document 2 (Japanese Patent Laid-Open No. 2005-311072, and Patent Document 3 (Japanese Patent Laid-Open No. 2007-67454).
Patent Document 1 discloses a technique which involves using a nitride substrate ((0001)-plane GaN substrate) which uses a polar plane to epitaxially grow a GaN layer, cleaning the GaN substrate with a reactor pressure set to 30 kilopascals, growing a 1-μm-thick first n-type GaN buffer layer with the substrate temperature maintained at 1050° C. and the reactor pressure maintained at 30 kilopascals, subsequently stopping raw material supply once, and further forming a 1-μm-thick second n-type GaN buffer layer by heating the substrate to a substrate temperature of 1100° C. with the reactor pressure maintained at 30 kilopascals. This crystal growth method allegedly provides a semiconductor apparatus having buffer layers with excellent surface flatness and good crystal quality.
Patent Document 2 discloses an invention of a light-emitting element produced by removing organic and other contaminations and moisture adhered to the surface of a GaN substrate while improving surface crystallinity of the substrate by flows of hydrogen gas, nitrogen gas, and ammonia gas, forming a multilayer structure intermediate layer made up of GaN and InGaN layers on the GaN substrate using flows of nitrogen gas and hydrogen gas, and then forming a reflective layer, active layer, and gallium nitride-based semiconductor layer on the intermediate layer.
Example 26 in Patent Document 3 discloses an invention of a laser element produced by forming a 3-μm-thick n-type GaN buffer layer doped with Si on a GaN substrate and building a stacked structure on the n-type GaN buffer layer. Incidentally, the Patent Document states that a 300-Angstrom or thinner buffer layer formed at a low temperature of around 500° C. may be provided between the GaN buffer layer and GaN substrate.
However, surface flatness and optical characteristics of a nitride semiconductor crystal obtained through epitaxial growth on a substrate whose principal plane is the polar plane cannot be said to be quite sufficient, and growth conditions require further examination. Besides, a problem resulting inevitably from the use of the polar plane as a growth substrate has also been recognized. For example, there is a known problem with a quantum well active layer structure (e.g., InGaN/GaN quantum well active layer structure) formed on a c+-plane of a hexagonal system such as a c-plane GaN substrate. Namely, so-called quantum-confined Stark effect (QCSE) will decrease a recombination rate between electrons and positive holes, making luminous efficiency lower than ideal.
With these circumstances as a backdrop, attempts have been made to create a quantum well active layer structure on a substrate whose principal plane is a nonpolar plane. However, epitaxial growth on nonpolar planes of hexagonal III-V nitride crystals, i.e., on the a-plane, r-plane, and m-plane, is considered to be difficult, and in particular, high-quality, stacked, hexagonal III-V nitride semiconductor structures cannot be obtained on the m-plane at present.
For example, Non-Patent Document 1 (Abstract Book of the 66th Academic Lecture Meeting of the Japan Society of Applied Physics (Autumn 2005), 11p-N-4) reports a special crystal growth method which involves cutting stripes in a GaN substrate whose principal plane is the c-plane such that the m-plane will be exposed as a flank and epitaxially growing a nitride semiconductor crystal on the striped side wall surface (m-plane) while recognizing the difficulty of crystal growth on the m-plane by stating, “it is difficult to grow a crystal on the m-plane which is nonpolar”.
Also, Non-Patent Document 2 (Appl. Phys. Lett., Vol. 82, No. 11 (17 Mar. 2003), p. 1793-1795) reports that a GaN layer was grown on a ZnO substrate whose principal plane is the m-plane by a plasma-assisted MBE (Molecular Beam Epitaxy) process. However, it is stated that the resulting GaN layer was “slate-like.” There were severe surface irregularities and a single-crystal GaN layer was not available.
Furthermore, with the preliminary that an epitaxial growth interface is prone to defects when a crystal of a nitride semiconductor is grown on an m-plane substrate, Patent Document 4 (U.S. Pat. No. 7,208,393) describes a technique for reducing the defects, as follows. Specifically, a GaN buffer is grown on an m-plane substrate by an H-VPE (Hydride Vapor Phase Epitaxy) process. Then, the substrate is taken out of the reactor once to form a dielectric mask and create stripes on the mask. Subsequently, using the substrate with the mask formed as a new substrate, an epitaxial film is formed by the MOCVD process and flattened through openings of the stripe mask by lateral overgrowth (known as LEO or ELO).
Patent Document 1: Japanese Patent Laid-Open No. 2005-347494
Patent Document 2: Japanese Patent Laid-Open No. 2005-311072
Patent Document 3: Japanese Patent Laid-Open No. 2007-67454
Patent Document 4: U.S. Pat. No. 7,208,393
Non-Patent Document 1: Abstract Book of the 66th Academic Lecture Meeting of the Japan Society of Applied Physics (Autumn 2005), 11p-N-4.
Non-Patent Document 2: Appl. Phys. Lett., Vol. 82, No. 11 (17 Mar. 2003), p. 1793-1795.
Non-Patent Document 3: C. J. Humphreys at al., “Applications Environmental Impact and Microstructure of Light-Emitting Diodes” MICROSCOPY AND ANALYSIS, NOVEMBER 2007, pp. 5-8.
Non-Patent Document 4: K. SAITOH, “High-resolution Z-contrast Imaging by the HAADF-STEM Method”, J. Cryst. Soc. Jpn. , 47(1), 9-14 (2005).
Non-Patent Document 5: K. WATANABE; “Imaging in High Resolution HAADF-STEM”, J. Cryst. Soc. Jpn., 47(1), 15-19 (2005).
Non-Patent Document 6: H. Matsumoto, et. al., Material Transactions, JIM 40, 1999, pp. 1436-1443.